SKIN STRUCTURE
Generally stated, the skin consists of two layers that are completely different in character. The more superficial and thinner layer, the epidermis, is epithelial tissue that is derived from ectoderm. The deeper and thicker layer, the dermis, consists of connective tissue that is derived from mesoderm. These two layers are firmly cemented together to form a cohesive membrane--the skin--which varies in thickness from less than 0.5 mm. to 3 or even 4 mm. or more in different parts of the body. The skin rests on subcutaneous tissue which is sometimes called the hypodermis, but is not, like the epidermis, considered part of the skin. Irregularly spaced bundles of collagenic fibers extend from the dermis into the subcutaneous tissue to provide anchorage for the skin. The subcutaneous tissue permits the skin over most parts of the body to have considerable latitude of movement.
The epidermis of the skin is composed of stratified squamous keratinizing epithelium. Like all epithelium, the epidermis contains no capillaries, so that it is nourished by diffusion from capillaries that are in the deeper layer of the skin, the dermis.
Since keratin is continuously worn away or shed from the surface, it must be continuously added to from beneath by the changing of living cells into keratin. this requires that the living cells of the epidermis continuously proliferate to maintain their numbers.
Many processes are in more or less continual operation in the epidermis: (1) cell division in the deep layers, (2) cells being pushed toward the surface as a result, (3) cells farthest from the dermis being transformed into keratin and (4) keratin desquamating from the surface. If these 4 processes are not synchronized properly--and in many skin conditions caused by age, exposure to ultraviolet radiation or disease, they are not--the character of the epidermis changes greatly.
The innermost of the inner layers is composed of basal cells that sit on the basal lamina that separates the epidermis from the underlying dermis. All epithelial tissues have on their basal surface this continuous sheetlike extracellular structure in contact with the underlying connective tissue. In some epithelial tissues (e.g., the skin) subject to friction, the basal lamina is anchored to the subjacent connective tissue by small fibers of collagen called anchoring fibers.
In most epithelia, fibrils of collagen (reticular fibers) complexed with amorphous protein-polysaccharides constitute another layer beneath the basal lamina called the fibrous or reticular lamina. This is a considerably thicker structure. Three constituents--basal lamina, ground substance (a highly hydrated, gel-like substance comprised of glycosaminoglycan and proteoglycan molecules), and reticular fibers--form what is called the basement membrane. The collagen of the basal lamina is primarily of type IV and that of the subjacent reticular fibers is probably type III collagen. The thick fibers below this layer are known to be formed by collagen type I. In this specification the term basement membrane will be reserved for the thicker structures visible with the light microscope. In current usage, the terms basal lamina and basement membrane are frequently used interchangeably.
Basal laminae, therefore, are thin layers of specialized extracellular matrix that underlie all epithelial cell sheets (and tubes). They also surround individual muscle cells, fat cells, and Schwann cells (which wrap around peripheral nerve fibers to form myelin). The basal lamina, thus, separates these cells and cell sheets from the underlying or surrounding connective tissue. However, there is increasing evidence that basal laminae serve more than simple structural and filtering roles. They seem to be able to induce cell differentiation, influence cell metabolism, organize the proteins in adjacent plasma membranes, and serve as specific "highways" for cell migration.
The basal lamina is synthesized by the cells that rest on it. Although the precise composition varies from tissue to tissue, and even from region to region within the same lamina, a major component of all basal laminae as noted above is type IV collagen. Type IV proalpha-chains are unusual in having extra-long extension peptides that are probably not cleaved after secretion. These procollagen molecules do not form typical collagen fibrils, although they do become covalently cross-linked to each other. In addition to proteoglycans and fibronectin, which are important constituents of basal laminae, the large glycoprotein laminin has been shown to be a major component of all basal laminae studied so far. It consists of at least two subunits (220,000 and 440,000 daltons) that are disulfide-bonded to each other. Basal laminae undoubtedly contain may other proteins yet to be identified. The detailed molecular organization of basal laminae is unknown, although there is some evidence that laminin and proteoglycan molecules are concentrated along the inner and outer surfaces of the basal lamina, with collagen molecules sandwiched in the middle. See also, Briggaman, Biochemical Composition of the Epidermal Dermal Junction and other Basement Membranes, Invest.Dermatology, 78(1): 1-6 (1982).
Basal laminae have been shown to perform a surprising diversity of functions. The basal lamina may act as a selective cellular barrier: for example, the lamina beneath epithelial cells prevents fibroblasts in the underlying connective tissue from making contact with the epithelial cells, but it does not stop macrophages, lymphocytes, or nerve processes from passing through it. It is likely that the basal lamina plays an important part in tissue regeneration after injury. When tissues such as muscle, nerve, and epithelia are damaged, the basal lamina survives and provides a scaffolding along which regenerating cells can migrate. In this way, the original tissue architecture is readily reconstructed.
However, recent research on connective tissue has led to the conclusion that with the aging of the skin fundamental structural modifications occur, especially in the basement membrane. These problems are of special significance to skin-care cosmetics. Since the extracellular connective tissue matrix produces an environment in which cells perform their function, the physiological interaction between cells and extracellular matrix is one of the key elements for normal epidermal-dermal interactions via an intact basement membrane.
Beyth and Culp (Mech. Aging Devel. 29: 151, 1985) point out that the significant physical and chemical modifications observed in the aging process are a consequence of a modified extracellular matrix. Pieraggi et al. (Virch. Arch. 1985) found a shift of the physiological equilibrium between skin fibroblasts and the extracellular matrix in aging skin. Sengel (Development Mechanisms, A. R. Liss, New York, pp. 123-135, 1985) points out the significance of the extracellular matrix, including the intact basement membrane, for the transmission of morphogenetic signals. The disturbance of the normal interactions between the epidermis and the dermis in aged skin is also known from ultrastructural investigations of the basement membrane. In addition, sunlight (ultraviolet) is known to injure the skin, not only by causing sunburn in the epidermis and inducing pigmentation, but by inducing changes in the basal membranes and deeper layers (dermis) below the epidermis. These changes appear later as premature aging of the skin --wrinkling, mottling, change in suppleness of the skin (altered connective tissue), dryness and alterations in the blood vessels. Ultraviolet radiation may also be absorbed by and damage DNA in cells present in the skin. It is further implicated in causing skin cancer.
Therefore, one may conclude that damage or injury to the basal laminae would have serious consequences for the entire epidermal layer and could very well result in associated detrimental cosmetic implications.